In the town of Ogden Marsh, Iowa,
David, the local sheriff enjoys a baseball game. His deputy, Russell,
spots Rory Hamill, a local resident, entering the outfield holding a
shotgun. David is forced to shoot and kill the unresponsive Rory. David
and his wife Judy, the community doctor, begin to notice other town
residents exhibiting bizarre behavior. The next night, a local farmer
burns his house down with his wife and son trapped inside, killing them.

While investigating the discovery of a pilot's body in a swamp, David
and Russell discover a cargo plane submerged under the water.
Communication services are lost, and soldiers arrive to take all
residents to quarantine at the high school. Everyone is examined for
symptoms of infection. Judy does not pass the examination and is
separated from David. She explains her fever is due to pregnancy, but is
sedated anyway. She wakes strapped to a gurney
as the perimeter of the facility is breached by the infected. The
military personnel evacuate, leaving Judy and others behind. David is
nearing evacuation, but escapes and rejoins Russell. They rescue Judy
and her hospital assistant, Becca.